1. Field of the Invention
The invention relates to equipment for arc welding and, in particular, it deals with an apparatus for overhead submerged arc welding.
2. Description of the Related Art
A large volume of operations in the manufacture of welded structures is carried out in welding rotatable annular joints of hollow structures with a restricted access to joints being welded on the inner side of the structure. Such joints include annular joints of closed vessels, annular joints of pipelines, tanks, casings, field welds and shell plating seams of ship hulls; longitudinal joints of large-area products which cannot be positioned to facilitate welding. They also include joints of difficult-to-position webs, segments, three-dimensional and planar sections, and the like.
A submerged arc overhead welding method is characterized in that a consumable electrode and a welding bath are turned at 180.degree. in comparison with downhand welding. Flux and electrode are fed to the work from bottom up, i.e. as though towards a ceiling, the electrode being supplied through a compacted flux.
This welding method is referred to hereinbelow as overhead submerged arc welding.
This welding method is referred to as overhead submerged arc welding also because the arc is in the body of metal.
So called welds are produced as a result of such welding.
Overhead welds may be of different types, e.g. penetration overhead welds, sealing overhead welds. There may be one-pass overhead welds and other types of overhead welds.
The penetration overhead welds are welds which are first to be produced in welding of a joint and which are located in the top part of sections being welded on the joint side opposite with respect to the electrode supply. Further welding of the joint, i.e. producing the necessary subsequent welds is carried out by any appropriate known method, the electrode being supplied on the same side as is the case with welding of the overhead penetration weld, e.g. the inner penetration welds of rotatable annular joints of vessels, tanks, joints between bottom sections of shell plating of ships and other structures.
The overhead penetration weld arc welding allows to a large extent the operation of welding from the interior of a vessel to be dispensed with in welding rotatable annular welds, and welding in confined spaces in producing straight welds of structures with a restricted access on the side of a ceiling can also be eliminated.
The sealing overhead welds are welds which are first to be produced in welding a joint and which are located on the bottom part of sections being welded on the electrode supply side of the joint. Further welding is carried out by any appropriate known method with the electrode supply on the side of the joint being welded opposite to that used in the overhead welding.
In practice, the penetration overhead welds are produced in welding annular and longitudinal joints of structures with a restricted access to joints being welded from the inside, the sealing welds being produced in welding longitudinal joints of difficult-to-position products, e.g. plate structures made out of segments, and other products.
One-pass overhead welds include welds produced in welding joints of a limited thickness arranged over the whole welded section. No further welding of the joint on any side thereof will be required.
Welding of sealing and one-pass welds poses many problems concerned with the formation of the surface of a finished weld.
During the overhead welding, the welding bath metal formed during arcing by fusion of the metal being welded, electrode material and flux is retained by a crust of partly melted flux and also by means of forming members. The forming members may be of various shapes and size and may be made, e.g. on the form of plates, backings, bars, sliders and other members and structures.
Flux is pressed against the welding zone positively from bottom up and, as flux is being consumed, its stock is continually replenished. For forming the part of a weld, flux may be supplied both on the bottom side, through the gap between the edges of a product being welded, and from the top by any appropriate known method when a filled layer of flux is formed. Special forming backings of flux holding means may also be used.
Numerous problems arise especially in welding hard to get at joints of large-size products of cylindrical or like configuration such as ship hulls and boiler units where especially high quality of welds is required and where the products must be rotated about their axis during welding operations, as well as in welding large-area planar products which it is very difficult to place in a position facilitating the welding.
Known in the art is an apparatus for overhead submerged arc welding (SU, A, 1348111), comprising a driven trolley supporting a vertical slide having a carriage. The carriage supports an arm and a self-aligning forming means, a hopper containing flux, and a copying member are provided at the end of the arm by means of a pivot pin. A bowl with a nozzle extending therethrough is provided in the hopper, in the top part thereof, and a welding head is mounted directly on the hopper. In addition, an auger feeder communicating with the bowl and having an auger rotation drive is mounted in the hopper.
The construction of this prior art apparatus allows both absolute flux pressure value in the bowl and force with which the forming means is pressed against a work being welded and the ratio between them to be varied.
This apparatus allows welding to be carried out with the desired weld forming on either side over a broad range of process capabilities with a large range of welded products.
However, as the forming means and copying member of this apparatus are mounted on one and the same pivot pin, the work can force the forming means away in case of a substantial convexity of the joint being welded so that position of the axis of rotation of the hopper and copying member with respect to the surface of the work being welded undergoes a substantial change. This results in an abrupt change in flux pressure in the bowl as compared with present pressure values at various points where welding bath is formed (upstream of the arc, in the area of the arc, in the area of the welding bath and in the solidification zone of the welding bath), i.e. in disruption of the welding process as a whole and, hence, in impaired quality of the welded joint.
In cases of a substantial concavity of the joint being welded on the underside of the joint the forming means is separated from the surface and a substantial additional clearance forms between the working face of the forming means and the work so as to disrupt the welding process as well.
In addition, the fact that the hopper with the bowl is supported by means of a common pivot pin mounted on the arm of the carriage of the vertical slide in such apparatus cannot allow the bowl to swing in its transverse plane which results in fluctuations of flux pressure in the bowl during welding.
Also known in the art is an apparatus for overhead submerged arc welding (DE, C, 3430394), comprising a driven trolley supporting a vertical slide having a carriage, a forming means mounted for swinging in its longitudinal and transverse planes on an end of an arm supported on the carriage of the vertical slide, and a hopper containing flux mounted on a pivot pin and having a welding head with a nozzle for a consumable electrode, a bowl provided in the top part of the hopper, the end of the nozzle for a consumable electrode extending through the bowl, and means for supplying flux and pressing it against the work. The pivot pin of the hopper is mounted on an arm which is mounted for movement in the direction towards the work being welded. The forming means is supported by the arm by means of an individual pivot pin. The arm also supports a copying member which is engageable with the surface of the work being welded during welding.
The carriage drive and the vertical slide are mounted on the driven trolley for moving the whole apparatus in the welding direction.
This apparatus allows a permanent contact between the forming means and copying member on the one hand and the work being welded on the other hand to be maintained during welding under various fluctuations of parameters of assembly before welding and geometry of the joint being welded (e.g. misalignment of plate edges, convexity or concavity, undulations, clearances, and the like).
Therefore, irrespective of any changes in position of the forming means caused by a change in profile of the surface of the work being welded at the point of engagement with the forming means, the copying member remains in permanent contact with the work surface thereby preventing the forming means from affecting the position of the hopper containing flux. This facility allows undesirable oscillations of the hopper under fluctuations of profile of the work surface to be eliminated, and welding parameters such as thickness of a flux backing and flux pressure in the welding zone can be stabilized.
The flux backing is an area of a compacted compressed layer of flux in which a preset pressure distributed over the whole area of the flux backing obtains and which is located between the top surface of the bowl having its open part facing towards the work being welded and the surface of the work being welded, which exerts pressure upon the surface of the joint being welded in the welding zone.
The suspension pivot of the forming means in the prior art apparatus may be made in the form of a point-like abutment at the end of a double-arm lever. The fulcrum of the lever is attached to an arm, and the other arm of the lever is connected to a power actuator which is pivotally connected to said carriage arm. The copying member in the form of a wheel is mounted on the pivot pin of the hopper.
This construction of the apparatus provides conditions for a smooth copying of the surface of the joint being welded by the forming means to maintain stability of preset flux layer pressure during welding.
In one embodiment of the prior art apparatus, the suspension pivot of the forming means is in the form of a point-like abutment at the end of an arm provided adjacent to the welding nozzle. The copying member is in the form of projections on the surface of the forming means. The suspension pivot of the hopper is located on the side of the forming means opposite to the nozzle, and the hopper is provided with a means for moving the hopper with respect to its suspension pivot.
This construction of the apparatus makes it possible to provide conditions for a smooth copying of the surface of the joint being welded by the forming means and to adjust position of the hopper with respect to the work being welded which is necessary in welding works having different geometry.
In carrying out welding with this apparatus, it is possible to ensure automatic conduct of the welding process in producing overhead welds and to obtain high-quality welded joints owing to the creation and maintenance of preset flux pressures at various points along the joint being welded (upstream the arc, in the area of the arc, in the area of the welding bath and in the area downstream the welding bath and up to the formed weld).
This apparatus allows high-quality welding of overhead welds over a broad range of process capabilities to be carried out. However, the fact that the hopper with bowl, welding head, auger feeder and other components are mounted in the prior art apparatus on a pivot pin supported by an arm of the carriage of the vertical slide cannot ensure swinging of the bowl in its transverse plane, whereby deviations from present flux pressure in the welding zone and in the areas of formation of the welding bath occur in welding works with substantial local fluctuations of geometry and joint assembly, (especially with large alternating misalignments of edges of the joints being welded as well as in cases of tapering joints, e.g. in welding bottoms of railway tanks, boilers, vessels, and the like having one-sided misalignments of weld reinforcements at points of T-crossings of longitudinal and annular welds and quality of overhead welds is impaired as a whole.